In the drive trains of motor vehicles, starter clutches or hydrodynamic torque converters are customarily used as the starter elements. Especially when a hydrodynamic torque converter is used, a lubricating-cooling concept for an automatic transmission is laid out such that the hydrodynamic torque converter and a lubricating oil circuit are supplied by a hydraulic pump via a secondary circuit of a hydraulic system in the transmission. Shifting components, which are provided for adjusting the gear ratio in the transmission, along with pilot elements in the transmission, are supplied with hydraulic fluid via a primary circuit by the hydraulic pump.
A prioritization of the primary circuit and the secondary circuit is performed via a so-called main pressure valve or a system pressure valve, which is arranged between the hydraulic pump and the primary circuit or the secondary circuit, and distributes the volume flow of hydraulic fluid forced by the hydraulic pump to the two circuits as needed.
The hydraulic fluid that is fed to the secondary circuit, via the main pressure valve, is first fed to the hydrodynamic torque converter. The hydraulic fluid then flows through an oil cooler for cooling it, after which it is used to lubricate the transmission.
In place of the hydrodynamic torque converter, if a friction clutch is used as the starter element, the volume flow of hydraulic fluid that is required for cooling is fed back to the hydraulic fluid reservoir after flowing through the starter clutch and thus cannot be used to lubricate the transmission. Thus, if the hydraulic pressure pump should have decreased pump performance, under certain circumstances an insufficient supply of lubricant to the transmission may result.
Furthermore, known-in-the-art hydraulic systems, via which hydraulic fluid is pumped to a cooling system for the starter clutch, a radiator for the hydraulic fluid and a lubricant supply for the transmission, are designed such that in order to limit the volume flow through the oil cooler a branch line is provided, via which excess hydraulic fluid can be diverted to a hydraulic fluid reservoir, if necessary. The limitation of the volume flow through the oil cooler is provided to prevent damage to the cooler caused by excessive pressure levels.
The disadvantage associated with this is that the volume flow that is diverted to the hydraulic fluid reservoir in order to prevent excessive pressure levels in the cooler is not fed to the cooling system for the starter clutch or to the lubricant supply, so that, especially if the performance of the hydraulic pressure pump is low, these two areas of the secondary circuit will not be supplied with the necessary quantity of hydraulic fluid.
The object of the present invention is to provide a hydraulic system for a transmission with a starter clutch, with excessively high pressure levels in a radiator for a hydraulic fluid, are prevented while, at the same time, the supply of hydraulic fluid to the cooling system for a starter clutch and to the lubricant supply for the transmission are guaranteed even if the hydraulic pressure pump is operating at decreased capacity.
According to the invention, this object is attained with a hydraulic system pursuant to the characterizing features of patent claim 1.